text:
Do airports archive the registration numbers of arriving and departing flights, and if so, for how long?
I am researching a flight I took as a child from LAX to Copenhagen in 1960 in a SAS DC-7C. I know the flight number, route, stops, departure time and date and wish to identify specifically which of SAS's -7C's I was on- either by the Danish registration number or the plane's "viking designator"- Frede Viking, Rolf Viking, etc.
I know this is a long shot- but my question is: Do airports archive the registration numbers of arriving and departing flights, and if so, for how long?
By the way I am in touch with both SAS's corporate office and their museum in Oslo, Norway on this matter.

I'm sure that airports keep records for tax and regulatory compliance, and that they will keep these for a number of years. I can't see why airports in general would keep them for sixty years. However, what 'airports' do is not really pertinent to your problem. It's the records kept by the airports visited by your aircraft on that flight that interest you and only the airports involved would know what records they have. You should contact them.

I would guess that you are more likely to get an answer by contacting SAS directly than by getting in touch with the airports. Since you've done that, I'd say you're on your way, and it would be cool if you would stop back by to at least let us know if they had the info.


text:
Will optimising the weight distribution of a commercial flight save fuel?
Since the centre of gravity must lay in a certain location, I'm wondering if it would save fuel if it lays exactly in the right spot?
Let's assume that you have an A320 and weight distribution is fine (within tolerance). If ballast is shifted until COG is located at the perfect location, would you save fuel? How much would that be?

Shifting the fuel during ascent descent and level flight was an active task on the Concorde

For precisely this reason the A310-300 had a fuel tank in the horizontal tail. However, the pilots I spoke with (Pan Am) did not understand the purpose and insisted that the tank is empty all the time. The A330 and A340 also have a tail fuel tank, and here its fuel level is automatically controlled.

A330 tank arrangement (picture source). Note the trim tank in the horizontal tail.
Shifting the c.g. a little back reduces drag slightly (less than 2% reduction in fuel consumption, depending on initial c.g. position) but also static stability. This is no problem when the FCS adds enough stability, but in case of a system failure you should make sure that the rear tank can be emptied soon. Hand flying an aircraft with relaxed static stability is tiring.
The A350 did away with this feature because it was felt that the added complexity would outweigh the benefits.

the pilots I spoke with (Pan Am) did not understand the purpose Are you saying they were not aware that it was for CG purposes, or that the benefit was so miniscule they didn't see a point in having it there?

How would 2% less drag reduce fuel consumption by 2%?

@TomMcW: They were convinced that the aircraft would be totally unstable as soon as fuel would be transferred to the rear tank. This was at a time when you could still walk up and ask to visit them in the cockpit. Which I did. I did not, however, try to explain the benefits to them, seeing how convinced they were of their wisdom.

@dani: Thanks, this was sloppy of me. At those small values the error is not very large, however.

Sad to say, I found through the years prior to my retirement in 1999 that many pilots did not understand weight and balance. I also remember running across two FAA POIs (Principal Operations Inspectors) that were in charge of monitoring airline compliance with weight and balance standards that lacked basic knowledge of the subject. One even expressed the opinion that he was good at weight and balance. The other was more realistic, asking me personally at the end of a 2 hour presentation if I could explain the "teeter-totter problem."

Short answer: yes it can.  Long answer: Yes, but this may not always be practical for a given passenger manifest and cargo.  On a dynamically stable airplane, a aft CG is more beneficial, since it reduces lift generated by the tailplane, thereby reducing induced drag on the wings.

source: Stack Exchange

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sort: votes


text:
What are the correct names of the parking spaces for turn-around?
A plane can be parked at the terminal and the passengers can board through a jet bridge. Also it can be parked somewhere else, far away from the terminal and the passengers would need to take the bus to get there.
What is the correct name of those parking spaces?

They are referred to as aircraft stands.
ICAO Definition:

Aircraft stand.
  A designated area on an apron intended to be used for parking an aircraft.

Stands at the terminal building are commonly referred to as gates, while stands further away may be referred to as remote stands or remote gates. The aircraft stand term covers both types.

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text:
Why do engine cowl lips appear larger and more blunt on the sides than on the top and bottom?
I was looking into engine nacelles. I was not able to understand the design shape of it
Why is it that in most aircraft, the engine cowl lips on the side are bigger (more blunt) than on the top and bottom.

maybe it's just perspective? you see them more often from the side

It sounds like you are describing a specific feature of B737. It's engine nacelles are flattened at the bottom because the engines grew larger as the aircraft evolved and the ground clearance is therefore very limited. So they moved engine accessories (starter, gearbox, electric generator, hydraulic pump etc.) to the sides and flattened the bottom of the nacelles to increase it a bit.

For subsonic aircraft, the nacelle lips need to be blunt and well rounded to accommodate the different flow regimes encountered during flight. At take-off (right side of the image below), the engine will suck in surrounding air, so the stagnation line of the intake flow is well outside of the center of the lip, whereas in cruise (left side of the image below) only the center stream tube of oncoming air is ingested, and much of the air flowing towards the nacelle is spilled over the sides. Accordingly, the stagnation line is well inside of the lip's center. A sharper lip contour would lead to flow separation due to the small radius (and hence the rapid change in direction) the flow would have to endure.

Image from Aerodynamic Design of Transport Aircraft by Ed Obert, 2009
Added to this is the influence of angle of attack, which will shift the stagnation line up or down on the lip. Sideslip will shift it left and right. Nacelles are mounted far ahead of the wing not only for flutter suppression, but also to reduce the wing's influence on the intake flow. I doubt that the added variations of sideslip would be stronger than those of angle of attack, so I agree with @fooot that more bluntness on the side is unusual.
Earlier designs used less blunt lips and had problems with flow separation in the intake at low speed, which led to the addition of secondary inlet doors. Today, the intake is optimized with CFD and this means that the radius may be continuously changing over the circumference, due to the influence of wing and fuselage on the flow around the nacelle.
Here I have to disagree with @fooot, because accessories are mounted close to the engine, and intake lips are quite a bit ahead of the engine face in order to equalize the flow. The flow speed at the engine face is around Mach 0.4 to 0.5, so the intake has to accelerate or decelerate the flow quite a bit, without separation or much cross flow. A lot of that happens already outside of the intake, but still needs some intake length, and most accessories are so far away from the intake lip that they do not influence it's thickness.

added a picture of the streamtubes for the two conditions that you mentioned, to make it a bit easier to understand, hope you don't mind.

@ROIMaison: Not at all, thank you very much!

Images of 767 and 777 aircraft that I looked at show pretty uniform lips all the way around. This 757 engine (image below) looks thinner on the sides, which is the opposite of what you have seen.

The shape of the cowl (and lip) is going to depend mostly on the shape of the nacelle. The 757 engine (image above) has engine accessories at the bottom of the engine, which requires a thicker nacelle to cover, which requires a larger radius on the cowl lip to streamline. As discussed in this question, the 737 engines have a unique shape to provide ground clearance, so the accessories are pushed to the sides.

The configuration shown in this picture definitely runs a risk of foreign object ingestion...


text:
What are the rules for driving at a public, uncontrolled airport in the USA?
Are there any rules for driving a passenger vehicle on taxiways and/or runways at an uncontrolled airport in the USA?

The FAA has issued an FAA Guide to
Ground Vehicle
Operations -
A Comprehensive Guide
to Safe Driving on the
Airport Surface which covers what they call "non-towered airports"

NON-TOWERED AIRPORTS
  When the control tower is closed or if there is no tower, the airport
  is referred to as non-towered. At a non-towered airport, you do not
  need controller permission before entering a runway or taxiway.
  Below are some best practices for operating on a non-towered
  airport:

When you approach the runways and taxiways, STOP, LOOK
  both ways, and LISTEN for aircraft that are landing or taking
  off. Vehicle windows should be open to do this properly.
Alert others when you are using a taxiway or runway by
  always making an announcement on the radio before you
  enter. Be specific with your location and intentions.
Always yield the right-of-way to taxiing aircraft and give them
  plenty of room. If an aircraft is headed toward you on the
  same taxiway, move out of the aircraft’s way.
Always carry a radio tuned to the airport’s Common Traffic
  Advisory Frequency (CTAF) or UNICOM.
If an aircraft is about to land on a runway that you need to
  cross, stop well clear of the runway. Continue to yield to the
  aircraft until it has landed and taxied off of the runway.
Be aware that some
  aircraft at non-towered
  airports are not equipped
  with radios.
Before you cross a runway, ensure that no potentially
  conflicting aircraft are taxiing, landing or taking off. Be
  aware of aircraft at non-towered airports that frequently make
  touch-and-go landings (immediately after landing, full power
  is applied and the aircraft takes off again).
If your vehicle has a rotating beacon, be sure to turn it on
  anytime you are on the airport surface. Turn on headlights as
  well, being careful not to blind any pilots in the area.

This is clearly very good advice in any jurisdiction. Are these rules enforceable in law at all? Also the OP specifically mentions "passenger vehicle", which makes me wonder whether his intent is strictly in the area of the "ground operations" intended by the FAA...

@IanF1: My guess is that US state law concerning traffic offences (e.g. careless driving) does not apply on private property. However, if you cause injury or damage, I imagine it is likely a court will take into account the terms of any agreement, between a driver and an airport owner or operator, under which the driver is permitted access. This is just a guess though.

@RedGrittyBrick I asked about uncontrolled airports.  Your answer seems to cover "non-towered airports" does this mean that the rules are the same for class E and class G airports?

@SpongeBob: My understanding is that the FAA guidance to ground-vehicle drivers at non-towered airports applies both when the airspace immediately above ground level is controlled (Class E) and when it is uncontrolled (Class G).

I actually work at a "non controlled" (no tower) airport in southern Arizona. I strongly advise not to drive on the runway.
I recently made that mistake.
I made sure we were safe, there was no traffic in the pattern and took all necessary precautions to ensure my brother in law would be safe as he made 4 passes on his motorcycle.
However, unbeknownst to me, doing so is highly illegal! It's no laughing matter. I now have to answer to the local police and the FAA.
If you are insisting on doing a run, go talk to the Airport Manager first.  I mistakenly asked the F.B.O. personnel for permission and not the site manager.
Some places will let you apply for a permit to do curtain things at an airport.
DO NOT DRIVE ON THE RUNWAY UNLESS YOU HAVE A PERMIT OR DIRECT PERMISSION FROM  THE PERSON / PEOPLE WHO RUN THE FACILITY. IT IS A FEDERAL CRIME TO DO SO WITHOUT PERMISSION!)

Welcome to aviation.SE. Could you please consider editing your answer to improve it? Adding a bit of formatting (paragraphs and such) helps with readability. Plus, we tend to prefer strong statements to be sourced, do you have a reference to confirm that, as you say, "it is a federal crime"? (also, please avoid using all caps, it is considered a bit rude on the internet, and thankfully we do not write NOTAMs here).

I edited your answer a bit; while your experience is useful in the answer, please provide a link to or a citation of which code/law/regulation is in play for a crime to be committed.  That will massively help your answer.

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text:
Why do multiple planes arrive from the same airport at the same time?
I'm looking at the Arriving flights page of the Vancouver Airport and in many cases there are several planes coming from the same airport, arriving at the same time, all at the same gate.
Does it count for only one plane and they are affiliated companies?
Here are some screenshots:

https://en.wikipedia.org/wiki/Codeshare_agreement

This has to do with codesharing or codeshare. Two or more airlines can agree to share the same route and operate it with only one aircraft to reduce cost of operation.
See the WikiPedia article on codeshare
Here an excerpt:

A codeshare agreement, sometimes simply codeshare, is an aviation business arrangement where two or more airlines share the same flight. A seat can be purchased on one airline but is actually operated by a cooperating airline under a different flight number or code

In the case where multiple flights are listed arriving at the same time from the same departure airports, there will only be one physical airplane serving the connection but under different flight numbers and operators. The different flight numbers and operators are listed to make it easier for passengers to identify their flight if codeshares are being used.
On busy airports or where there is not enough space on the information displays to show all flights of a codeshare, the relation is only shown once with rotating flight numbers.

As others have already said, those are codeshare flights. Just to clarify, this means that there is only one physical plane flying this route at this particular time but it has several flight numbers for marketing reasons (it can also make a difference for things like frequent flyer miles, etc.)
Often, a three-digit flight number means the relevant airline is the “real” (or “operating”) carrier, whereas longer numbers are used by “virtual” (or “marketing”) carriers. Thus the first flight in your list would actually be operated by Lufthansa, the next one by Virgin Atlantic, etc.
There are several entries on the arrival and departure list because different passengers will expect a different airline name and flight number (the codes like LH492 or AC9101) and need to be able to find it in the list. In some airports, there is only one line on the video displays in the terminal, with the flight number rotating every 30 seconds or so.

"Often, a three-digit flight number means the relevant airline is the “real” (or “operating”) carrier," incorrect, number of digits in the flight number has nothing to do with it. It's purely airline dependent (e.g. KLM uses 3 digit numbers on intercontinental flights, 4 digit numbers on European flights).

@jwenting Indeed, which is why I used the word “often”…  What's incorrect on the other hand is claiming that it has nothing to do with it (quite an absolute statement!) when in fact many airlines do follow this convention.

Just because some airlines do it like this, it's got nothing to do with it. There's no IATA or ICAO rule about it, which is about the only thing that would cause it "to have something to do with it". Might as well claim that internal flights always have 4 digit numbers and intercontinental flights 3 digit numbers, after all some airlines do that...

@jwenting “Just” because some airlines do it, it's got something to do with it, it's that simple… I am describing a (probabilistic) fact, I never implied there was a rule that mandate it, I would not write that it's often like that if it was the case.

As SentryRaven correctly answered, they are codeshares.
The reason is mainly that passengers want to buy tickets for the whole trip from a single company, because it is easier and because it makes it clear responsibility of that airline to rebook them on another flight if they miss a connection due to delay of previous flight.
So to be able to offer better choice of connections, multiple airlines agree to sell tickets on the same flight and the flight then gets a separate number from each of them. It is especially common on short flights.

Airlines can sell tickets on each other's flights without codeshare.

@Relaxed: They can, but it would then still be ticket of the airline operating the flight, no? In which case the situation in case of need for rerouting would still be complicated.

Codesharing and airline alliances is the way around old flag carrier policies. Politicians see it as a sign of manlyhood for their nation if they have a flag carrier. Therefore, many more airlines are kept alive without an economical reason. Just look at Alitalia for example. With codesharing and alliances they can almost operate like one big airline.

@PeterKämpf: Well, flag carriers are still often the only way to operate in the air transport market which is still heavily regulated in big parts of the world.

It's complicated either way but as far as I know, cooperation (selling tickets, going around flag carrier policies, etc.) is perfectly possible with interline agreements. Codesharing seems to be about marketing.

@Relaxed a through ticket on the same airline gives the customer different rights as compared to changing nominal carrier. This is especially important in case of delays and cancelations where having all legs on the same airline means you can get compensation in case you miss your connection, were they different carriers you are not entitled to compensation.

@jwenting What makes you think that? You are definitely not entitled any compensation if you booked the two tickets separately, even on the same airline. But if you booked the journey as one ticket (from one of the airlines or from a travel agent), you might be covered even without codeshare (as opposed to a mere interline agreement).


text:
Looking for the correct propeller shape to create a small copter
I am trying to create a mini flying quadrocopter using a Raspberry Pi (mini programmable computer). So far I have hooked up the RPi to 4 motors with controls to each. I'm pretty happy with that side of it.
The part I need help with is the design of the propeller; my motors are not massive and I want to ensure I get the best design for maximum thrust to weight as well as manoeuvrability.
What do I need to look for in propeller design considering my limited motors?

May I respectfully disagree with the categorization as a duplicate? All linked answers have nothing on designing your own propeller. This aspect is genuinely new in this question.

@PeterKämpf It's annoying that the stated close reason is just the most popular one. As I recall, I voted to close as too broad but the close message misleadingly states that I marked it as a duplicate.

Hello A.White, and welcome to Aviation.SE. Do you have the technical specifications of the motors? It may help to know what kind of power they are able to provide, what their optium rotation rate is. Also the estimated total weight of the vehicle is of interest.

Use Mark Dela's XROTOR - it will give you the chord and twist distribution over span for a given speed, torque and thrust (well, two out of three).
It is based on Larabee's theory (PFD!) for a minimum induced loss rotor.
For a practical solution try to design an optimum propeller for the speed and torque of your motors and then try to find something similar in a  hobby shop. Building the propeller yourself will be a major effort - just balancing them well will not be easy.
If you are into doing everything yourself, try to carve a master and make a mold, maybe from silicone rubber. Then you can pour as many propellers as you need. Put glass- or carbon fibers into the mold and fill up with thickened epoxy. For thickening use a mixture of cotton short fibers and Aerosil 200.
The links to the materials are only for background information - I cannot endorse the sources because I have never ordered from those specific suppliers. The method in general will work, though.


text:
Do flight attendants have a regulatory responsibility to risk their own lives to save passengers?
A documentary about the collision at Tenerife seems to imply that the Pan Am flight attendants left the aircraft without helping a single passenger?  Is this correct? If so how is this possible? Are flight attendants required by regulation to place their own lives at risk to save passengers?

"but even as a passenger I'd like to think I'd try to get a few people out the door before jumping myself" The unfortunate reality is that situations involving life or death, especially your own, makes you behave in a manner that is much more about self preservation rather than altruism for people you really don't care about. Even if you later regret your actions. The crew of the Concordia (boat) were guilty of the same thing, there are a lot of historical references from people in similar situations.

Are you asking about a regulatory responsibility or an ethical responsibility?  If the first, I think this question is on-topic, if the second, possibly one of the faith-based Stacks would be a better fit.  For now, I'm going to edit your question on the assumption you meant the first. If not, please feel free to rollback.

In this modern world I would be very very surprised if it were possible to make a regulation or company policy forcing people to die. Having said that, they are there to assist in an emergency so unless they feel in imminent danger they do have responsibilities.

The actual extent of the duty varies by profession. When undergoing first aid and CPR training for educators, I was informed that my duty to rescue a student applied only to the extent that I could do so without endangering my own life. If a student was trapped under live power lines, unconscious in a room full of poison gas, or held hostage by people with guns demanding political concessions, I was supposed to get out of there and call the local emergency number (e.g. 911) rather than try to be a hero and end up as a second casualty with no one left to call for help.

In training, flight attendants are given lots of emergency training, but always with the caveat that they are under no obligation to put themselves in danger.  Emergency procedures should proceed up until the time a flight attendant feels they are in danger, then they are to evacuate the airplane themselves as quickly as possible.
There was a long standing joke among flight attendants when I was working, which was the most important emergency instruction you can give:  Follow me!  Funny, but not really.  It keeps things in perspective.  Not commonly known is that any emergency responder today operates under the same conditions.  You are not being paid to put yourself in danger beyond what your training will accommodate.

There are actually very few regulations in this regard, all of which are in 14 CFR 121.381 through 397.
Of those, only two pertain to the duties of a flight attendant. Most of the regulations are just about basic duties during planing/deplaning, but 121.397 is about emergencies:

§ 121.397 Emergency and emergency evacuation duties.
(a) Each certificate holder shall, for each type and model of airplane, assigned to each category of required crewmember, as appropriate, the necessary functions to be performed in an emergency or a situation requiring emergency evacuation. The certificate holder shall show those functions are realistic, can be practically accomplished, and will meet any reasonably anticipated emergency including the possible incapacitation of individual crewmembers or their inability to reach the passenger cabin because of shifting cargo in combination cargo-passenger airplanes.
(b) The certificate holder shall describe in its manual the functions of each category of required crewmembers under paragraph (a) of this section.

The specific part is (b), which basically means that the airline needs to develop SOP's (standard operating procedures/policies) and submit them to the FAA for approval. While this may be somewhat of a standard across operators, it is up to the airline itself to determine the extent that crew members are required to assist in an emergency.


text:
How can I use XFLR5 on a shape with no maximum thickness at one point?
I want to run an analysis with XFLR5 on a shape that is not really airfoil shape since it has no maximum thickness at one point only, it is basically like the shape below:

But when I run an analysis, the program keeps showing that at all angles it does not converge. Could the problem has to do with the XFLR not knowing the maximum thickness and at what position? or is there another possible reason?
Here are the coordinates of the shape:
http://www.filedropper.com/elipse

i uploaded the file

XFLR5 uses XFOIL to perform airfoil analysis. From the User Guide:

XFOIL is a (...) program for the design and analysis of subsonic isolated airfoils.
The XFOIL code is not foolproof, and requires some level of aerodynamic expertise and common sense on the part of the user.

The most general equations to describe the motion of fluids are the Navier-Stokes equations. XFOIL doesn't solve the NS equations directly but uses panel methods instead. This makes it much faster than other CFD simulation tools at the cost of restricting the geometries it can analyze.
Normally an airfoil has the following properties:

Simple, smooth geometry
Smaller thickness than length
Sharp trailing edge

The problem with your shape is not the maximum thickness (a relative thickness of 20% is still OK) or the constant distance between the upper and lower side but the blunt trailing edge. To some extent the really large thickness at over 75% of the chord affects too.
Whitcomb's supercritical airfoil for example has a nearly constant thickness from 20 to 50% chord but can be analyzed without errors.
The panel method splits the airfoil in many small panels and tries to find a circulation for each one of them so that the air follows the airfoil geometry (normal speed to each panel equals zero). To get physically more realistic results other equations like the Kutta-condition have to be used. One of the assumptions for this equation is that only shapes with a sharp trailing edge can produce lift.
Trying to solve equations when the basic assumptions are not met normally doesn't work and that's why your calculations do not converge.
Note: I'm not an XFOIL power user and perhaps someone can tweak it to get good results, but I doubt it. While it is possible to analyze airfoils with a blunt trailing edge the shape above is very far away from being an airfoil.
TL;DR: XFLR5 can only analyze airfoils. Your shape has not a sharp trailing edge and it is therefore not an airfoil.


text:
How does the outflow valve work on a cabin pressurization system?
When you fill up a cabin with warm air the pressure of the cabin will rise.  An outflow valve controls the desired pressure. But with pressure rising doesn't the temperature rise because you're compressing air into a space? Is this why the outflow valve is there, to regulate the pressure and temperature? Does the outflow value open and close depending on the pressure during phases like climb, cruise, and descent. Or does the outflow valve shut completely once the cabin altitude reaches 8,000ft? But then wouldn't the air get too hot because it keeps getting shoved into the cabin?
Does it use the isothermal rule, if so then what is the work done?

When you fill up a cabin with warm air the pressure of the cabin will
  rise.

You're not filling up a cabin with hot air, you're allowing sufficient air (of any temperature) into the cabin to match the air flowing out. What is important is the flow of air (~10 pounds per person per minute).

An outflow valve controls the desired pressure. But with
  pressure rising doesn't the temperature rise because you're
  compressing air into a space?

No, the outflow valve makes sure that the pressure DOESN'T rise, it stays roughly the same (allowing for climb/descent).
The classic paradigm for aircraft pressurisation is a (toy) balloon with a pin-prick leak - as long as you keep blowing it up as fast as the air is flowing oyut the leak, it will stay inflated. The outflow valves use flow as a means of controlling the pressure (differential), which then ensures the requirement for sufficient flow of breathing air per passenger.

To maintain a comfortable temperature for the passengers, automatic
systems regulate the mixture of heat from the engines and cold from
the air packs. To maintain the pressure in the cabin equal to that at
low altitude, even while the airplane is at 30,000 feet, the incoming
air is held within the cabin by opening and closing an outflow valve,
which releases the incoming air at a rate regulated by pressure
sensors. Think of a pressurized cabin as a balloon that has a leak but
is being inflated continuously.
On the ground, the airplane is unpressurized and the outflow valve is
wide open. During preflight, the pilot sets the cruise altitude on a
cabin pressure controller. As soon as the weight is off the main
wheels at takeoff, the outflow valve begins to close and the cabin
starts to pressurize. The airplane may be climbing at thousands of
feet per minute, but inside the cabin, the rate of “climb” is
approximately what you might experience driving up a hill. It might
take an average airliner about 20 minutes to reach a cruise altitude
of, say, 35,000 feet, at which point the pressurization system might
maintain the cabin at the pressure you’d experience at 7,000 feet:
about 11 pounds per square inch. Your ears may pop, but the effect is
mild because the climb rate is only 350 feet per minute. When the
airplane descends, the pilot sets the system controller to the
altitude of the destination airport, and the process works in reverse.
The structural strength of the airplane determines how much
differential pressure the cabin can tolerate—a typical figure is eight
pounds per square inch—and the fuselages of new airplane designs are
pressurized and depressurized many thousands of times during testing
to ensure their integrity. The higher the maximum differential
pressure, the closer to sea level the system can maintain the cabin.
Federal Aviation Regulations say that without pressurization, pilots
begin to need oxygen when they fly above 12,500 feet for more than 30
minutes, and passengers have to use it continuously above 15,000. On
airliners that operate at altitudes well above that, regulations
require that everyone aboard be supplied with 10 minutes of oxygen in
the event the cabin pressure can’t be maintained, which brings us to
the dramatic scenario known as explosive decompression.
If the door blew off a jet at altitude, all the air in the cabin would
depart very quickly and a momentary thick fog would envelope the cabin
as the water vapor in the air condensed instantly. Loose articles
would fly around and foam rubber would burst as the tiny air bubbles
within it expanded. Within a couple of seconds, oxygen masks would
drop down from the overhead panels, and you would have to pull yours
toward you and place it over your mouth and nose. The act of donning
the mask tugs on a lanyard that starts the flow of life-sustaining
oxygen.

Source: Air & Space Magazine

Welcome to aviation.SE! This appears to be copied from an online article. It's fine to quote other sources, but please make sure you identify them (link to them) and use the quote formatting to indicate it's a quote.

Hey guys, I wish you'd clarify a bit on why my edit was rejected. I deleted the last 2 paragraphs because I thought they had to do with structural limits on pressurization and emergencies, rather than with the valve. What am I missing? I'm really curious.


text:
Should I record my helicopter and autogyro time in separate logbooks?
I have been flying autogyros for some years now, and I am out to do my license on UL helicopters.
I bought a new logbook to log all time in helicopters separately from autogyros. Is that correct?
In a few days I will also be taking some lessons in an R22. That's not even UL anymore. Do I have to get another logbook for all flying of non-UL aircraft? Would I log it all in the same logbook?

Which country are you adding about? But it's usually easiest and best to log everything in one logbook. And online logging tools make it really easy to track all your flights and still easily calculate and print out whatever totals you need.

I usually fly in Costa Rica where I have most of my time logged. I'll be doing my UL helicopter license in Italy, and the flight in the R22 will be in Spain... I usually use ZuluLog, but for authorities I need also a good logbook.

If you're flying in multiple countries, this question might be helpful. FWIW, I have a single physical logbook and an electronic one. For checkrides or whatever, I print a report from the electronic logbook and I also show (or am prepared to show) the examiner my physical book if he wants to look at it. If it's a report to send to someone, I scan my physical book and send the pages with the electronic report. That covers every scenario I've had so far, but you may have other needs.

In the United States, except for that time which is used for certification or currency, the FAA permits you to log your time however you like, or even not log it at all.  That means that you can do pretty much whatever you feel like.
That said, here are my personal recommendations in order of how much I support them:

Get a digital pilot logbook. Log everything in it.
Get a professional pilot logbook with plenty of space for various categories and classes of aircraft. Copy all of your records from your old logbooks into it. Archive your old logbooks in a safe location. Use your new logbook for everything from here on.
Get a professional pilot logbook with plenty of space for various categories and classes of aircraft. Don't copy all the records from your old logbooks into it, just carry forward the totals. Archive your old logbooks in a safe location. Use your new logbook for everything from here on.
Get one logbook for each category/class of aircraft you intend to fly. Log your time in whichever logbook is appropriate. Remember which logbook is which. Keep them all easily accessible rather than storing them in a safe location. Worry about whether you lost one. Have a complicated system of adding up your totals from various logbooks. Make it difficult to look up flights chronologically. Feel sad.

Option 4 is... amusing.

I use ZuluLog, but sometime at authorities I need to present my logbook :) 2. Would love to, because I would also 'correct' some thing I didn't logged 100% correct in the past, but have a lot of signs and stamps in my most used one 3. I think I'll stick to that option, and have parallel ZuluLog for easy access to totals of different ratings 4. Thanks for making me laugh! ;)

You would have to make sure your flight time was differentiated into category and class of aircraft.  Most logbooks have a blank category on their pages; just fill in ULTRALIGHT or UL in that column and add your hours that way.
Now some of those flights can be logged under the columns for PIC, Cross Country flight time, dual instruction, takeoff and landings, etc. as well in the new logbook and can count towards some hour requirements for another certificate.  Reference 14 CFR Part 61 for specific requirements

Until now I haven't been differentiating because I was only flying UL Gyroplane, where I've logged PIC, XC, and instruction time. And I cannot add anymore the UL column because the book is closed already to certain pages and authorities have copies of if. So should I log then everything not UL in a different book?Should I log the UL heli in the same logbook as the Autogyro UL?

You can keep separate logbooks if you like or it's easier; A lot of pilots get multiple logbooks over the span of their careers.

There are no regulations governing how many logbooks you may use, nor their exact construction.  If it is easier for you to manage your time with multiple logs, that is just fine.
If you need a flight review entry, you can always photo copy that entry in one book, and clip or tape a copy into a different book.
Personally, when instructing, I use a "critique sheet" which I review with the student at the end of the flight, and we both sign. That sheet becomes my log entry. For other types of flights I may use a traditional log book. While I have several, I avoid the large professional books, because of their size and weight.


text:
Does a large simulator bay have standard public address announcements?
We are simulating a "simulator bay" (with simulators) within a simulation and were wondering if there were any announcements made over the loudspeaker audible to people inside or outside the simulators.  Thinking of these large 5-simulator buildings like FlightSafety operates so anyone that spends a lot of time there (instructors, airline new-hires, someone headed there for recurrent training)... any help is appreciated. Thinking of things like:

Bay 5 is now available, bay 5 is now available
James Mahan, please report to bay 4 for your check-ride
Maintenance crew to bay 1, maintenance crew to bay 1

You're creating a flight simulator simulator? How meta.

Any simulation should be designed to be an immersive experience, whether it's a simple PC application, or a full motion flight simulator. Any stimulus that is not part of the simulation itself breaks that immersive experience. So …. no.

Vomit cleanup team to Bay 4, repeat, Vomit cleanup team to Bay 4.

The main problem is when you exit the simulator, you might still be in the simulator.  How will you know when you're back in the real world???  See, this is how The Matrix happened...

Any reason you can't do this purely visual, with monitors or other wall-mounted screens?

@Harper I suggest watching The Thirteenth Floor (1999)

My experience with the military is that loudspeaker announcements like this do NOT occur in simulator buildings.  They would be both distracting and largely unnecessary when there are land lines and published schedules available.
Besides the simulators themselves there are briefing and debriefing rooms where crews and instructors are discussing conduct of the events.  Frequent loudspeaker announcements would interrupt these discussions and negatively impact the learning process.
A simulator building is much closer to the environment of a classroom or library than it is to a busy airport, train station, or other public area where you would hear announcements of the type you describe.
Depending on how much realism you are striving for there are some distinctive sounds you will hear in a room full of full motion simulators. These range from the low frequency industrial hum of HVAC and equip cooling fans, to the higher pitched whine of hydraulic pumps, to the swoosh and thump sounds of actuating cylinders moving and hitting bump stops during more aggressive motions associated with unusual attitude or spin recoveries. Up at the control console it will be a bit more muted, but you will hear the crew ICS and radio communications over a speaker, stall warning horns, GPWS alerts, etc.

My answer applies to the 141 training programs I've been a part of; Michael Hall's answer is correct - there are no loudspeaker announcements made, with one rare but important exception:
"FIRE ALARM IN SIMULATOR BAY. EVACUATE. EVACUATE."
Of course, since almost any commercial-use building has fire alarms and fire drills, this isn't especially interesting for your specific case.

Good point with the exception!

Don't worry, it's just a simulated fire.

Actually, it's probably just a simulated drill!

@Mark Don't worry, it's just a simulated life.

Even then, the PA announcement in this case should not be for the trainees; on-demand intercom through the simulation audio system for emergencies should be a typical safety requirement, since there's no guarantee the trainees can hear the PA properly through aviation headsets. The building itself will have standard fire safety protocols and alarms, such that a PA system will not be necessary.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What safety differences exist between carrying laptops in checked versus carry-on baggage?
Recent rumors say that the laptop ban might be extended to flights originating in Europe heading for the USA. This made me think, if laptops are not safe for carriage in the cabin how can they be safe for carriage in the belly of the aircraft? Say that a notebook is short wired such that if it turns on it ignites the battery. Storing the laptop in your carry on luggage would result in a cabin fire which can be put out by the cabin crew. But if this same laptop is in a checked-in bag there is no cabin crew near it. How is this risk mitigated? Or what are the procedures in case of a belly fire?
Furthermore if a laptop is carried inside a carry-on bag it can be switched-on at the security checkpoint to verify that it is a working laptop.

You don't need a short circuit to get a Lithium pack on fire, mechanical damage will do. Have you ever seen how checked luggage is handled? It really is a recipe for disaster.

@Pavel Oh wait... Now I know why Trump is so afraid of laptops.

The problem with your question is it assumes the reason for the ban is the risk of battery fires. But how do you know it is the actual reason?

@HugoWoesthuis sounds like a problem that could be solved with a wall

@CaffeineAddiction Exactly haha

It's not about genuine laptops being safer as checked baggage. They're not, they're safer as carry-on baggage, where eventual fires can be identified, isolated and dealt with. That's why we have rules to make passengers carry-on lithium batteries more instead of checking them in. example
The reasoning explaining the ban is that an attacker might use the shell of a laptop to carry something on board. Turning on the laptop doesn't really prove much about it being genuine. You can take a Raspberry Pi that's size of a matchstick box, pair it with 2 AA batteries to create fake internals and  connect them to the original display. That'll make a gutted-out laptop appear to run for few minutes, enough to fool any quick inspection. Leaving rest of space and mass budget to hide something sizeable, like a bomb or a weapon.
That's why you can't circumvent the ban by removing the battery and taking a charger instead. It refers to "large electronic devices", not necessary ones with a battery.

That seems to be the good answer, but without any reference it can't be much upvoted, just trusting your opinion. Any authoritative document supporting your suggestion? For example, I thought explosives could be sensed by an X-ray scanner (tomodensitometer), so hidden explosives in a plastic case shouldn't be a problem.

@mins, something should be easy to come up by Google as it was all over the news. Basically CIA claims to have ran across some concrete plan that would likely pass the X-ray—and as can be expected of information obtained from covert operation, didn't disclose any details.

@JanHudec: Thanks. It seems to be to prevent fake laptops to be pressed against the cabin skin. In the cargo their location will be random, and the effect minimized. Possibly the cargo content can be better inspected too.

@mins I referenced airline policy about li-ion cells. I don't think any other references could be cited. I never claimed that airports have no means of detecting explosives inside a laptop, I only said that it creates a hidden compartment. It obviously creates a security risk, because that bypasses one inspection in the chain of many, the organoleptic inspection. Nothing more, but nothing less either.

I don't know the exact reasoning behind the possible ban of laptops/tablets in carry-on luggage. If the DHS' main concern is physical access to the device then it is an understandable move.
Possible battery fire of said devices is a completely different animal and might be a risk not fully taken into account during the DHS decision making process. As you said, a fire in the cabin is accessible and can be fought by the cabin crew, a fire in the hold means flooding the hold with Halon 1301 and hope for the best.
There is - currently - no definite answer to your question. Safety and security is, in many ways, like trading risks/benefits and in this particular case we don't know the details on the security side of the equation. Therefore I hope that safety and security risks will be well weighted against each other prior to making a final decision.

I'm by no means an expert, but I'd say that Halon can't help, because Lithium cells have everything within them which is needed for sustainable fire. That makes them indistinguishable. Furthermore, there will be tens to hundreds battery packs in close proximity which can get ignited by the first fire.

Halon will prevent the burning Lithium battery from setting anything more conventionally flammable on fire (such as the rest of the passenger's luggage, and the other passengers' luggage). The battery should burn itself out after generating a significant but not critical amount of smoke. A shipment of hundreds of Lithium batteries would be a different story.

... which leads me to think that the principal cost of forcing passenger's laptop computers into the hold will be economic. The inevitable consequences will be much more frequent small fires, halon releases, emergency landings, damaged luggage, travel disruption, planes needing expensive post-fire maintenance, and so on. There should be no directly-caused loss of life, though one can't rule out an accident caused in part by reaction to the emergency situation (or heart attacks).


text:
Can a pilot work flight engineer's station?
In kinda reverse of Did Flight Engineers progress to be pilots?
In three-crew cockpit can a person qualified as pilot work in the position of flight engineer? Does a flight enginner have different qualifications that are not covered by an ATPL, thus rendering a pilot unqualified for the position?

If the FE has been incapacitated, someone in the cockpit will be taking over the responsibility!

The pilot cannot just sit down in the flight engineer station.  First there is a different FAA flight engineer certificate that the pilot needs to obtain.  Secondly, the pilot would need to go through the airline's training program for that station.
Granted, a pilot who has flown that airplane before should quickly get those done.

FWIW, airlines that (still) have FE position tend to start out newhires in that position and then promote them as they gain seniority. So any pilot sitting up front, will have most likely put in the time to qualify as an FE at some point and it would be more a matter of getting them recurrent than re-certified altogether.

@habu, this is true but not everywhere. In the Eastern Bloc, FE (or navigator for that matter) was considered like a rather different occupation, and they progressed by moving to larger more complex aircraft rather than to the pilot's seat. (Though the latter was, of course, possible with proper re-training).


text:
What's the point of intercepting an aircraft?
I know that it's possible for military aircraft to "intercept" a civilian or foreign aircraft (and I'm well aware of the US interception procedures), but why would they be  doing it in the first place?
What does it actually prevent from occurring?
They can't remotely control the aircraft, so is there anything that they can do once they intercept it?

Remember KAL 007 ?

The most common reason for interception is that an aircraft is not communicating with ATC and is somewhere that they aren't supposed to be.  Often times it turns out to be unintentional with the aircraft on a VFR flight (i.e. poor flight planning or deviating for weather) and straying into a restricted or prohibited area.
In these cases, the standard intercept signals (AIM 5-6-4) are designed to attract the pilot's attention and have them proceed out of the area.  In addition to the "standard" signals, they can also drop flares in front of the airplane to really get their attention if needed.
In cases where the airplane still doesn't respond and the aircraft appears to be a threat to national security, the decision can be made by a few senior military and civilian officials to actually shoot down the aircraft.

For more detail, in the US section 5-6-2 of the AIM gives the following reasons that a civil aircraft may be intercepted by a military aircraft:

5-6-2. Interception Procedures
a. General.
1. In conjunction with the FAA, Air Defense Sectors monitor air traffic and could order an intercept in the interest of national
  security or defense. Intercepts during peacetime operations are vastly
  different than those conducted under increased states of readiness.
  The interceptors may be fighters or rotary wing aircraft. The reasons
  for aircraft intercept include, but are not limited to:
(a) Identify an aircraft;
(b) Track an aircraft;
(c) Inspect an aircraft;
(d) Divert an aircraft;
(e) Establish communications with an aircraft.
...

Flightgear.org also has a good wiki on interception procedures that covers some of the common reasons that they are used (and includes all of the ones that I was going to include):

Reasons for interception in real life
Civilian aircraft
While interception of civilian aircraft is a last resort, interception
  is often the only means available to identify an aircraft that have
  not filed a flight plan and/or have no transponder and can not be
  contacted. Apart from identification interception is as well often the
  only means to redirect an aircraft that is straying into limited
  airspace or is believed to be involved in illegal activities.
Identification
  Visual identification of aircraft that can not otherwise be identified.
Redirection
  An aircraft may be intercepted and through visual signals or radio communication on emergency channels be requested to
  change route and possibly to land at an specific airport if an aircraft

is straying away from a route,
are entering a danger, restricted or prohibited area,
are suspected to fly illegally or is smuggling goods or persons,
enters a countries airspace without permit an fails to follow instructions to leave the airspace or land at a specific airport,
enters a countries airspace at different positions or routes than permitted, or
is a hazard to other aircraft

If you've been reading the news in the past week, the RAF and German Air Force have been intercepting quite a large number of Russian jets over international water. Why are they intercepting in this case?
Isn't it legal to fly over international water as long as you don't harm any other aircraft?

@TaherElhouderi It is legal to fly in international airspace. However, if you're flying a bomber near the territory of a not-that-friendly country, they can send interceptors to remind you that you're being watched, and that you can't just slip in to national airspace without being noticed and possibly shot down. On the flip side, one of the points of military exercises near an unfriendly country is to evaluate their air defense based on their response to you, and remind them that you can send aircraft over to their territory.

Note that if the AIM-5-6-4 intercept signal becomes an AIM-7 Sidewinder, you've not responded properly to the earlier signals.

@TaherElhouderi - International waters and international airspace are different concepts. International maritime law establishes territorial versus international waters, but there are no international laws regulating the establishment of Air Defense Identification Zones or ADIZ. Aircraft entering an ADIZ without following proper procedure for the country establishing that zone will be intercepted; it's the purpose of having the zone in the first place. The only implicit agreement is that an ADIZ cannot overlap another country's ADIZ or its political land borders.

@cpast - Excellent point. Russian Bear bombers and U.S. F-15Cs have been playing this game for years; the Bears will intentionally violate the Alaska ADIZ to measure U.S. response times. During periods of higher tension between the U.S. and Russia, the U.S. will scramble LONG before the Bear reaches the ADIZ.

@KeithS, So basically Russia could make an AIDZ just off the coast of America and vice versa, and wouldn't count as violation of International law? (assuming the AIDZ doesn’t overlap the other countries airspace).

@TaherElhouderi -  https://upload.wikimedia.org/wikipedia/en/thumb/d/d8/Alaskan_ADIZ.jpg/1024px-Alaskan_ADIZ.jpg - That is almost exactly what has happened, and Russia tests this zone almost every day. There are some practical limits to the outer borders of an ADIZ as well as legal; the ADIZ is of no use if your radar can't see past the ADIZ borders (unless you spend considerably more money and man-hours using military aircraft patrols).

Military interception usually happens after failure to make the aircraft comply by other means (such as radio, light signals, flares). So, first and foremost it is one way to do visual communications, which is usually hard for the pilot to miss. The messages for follow (wide level turn), land (low pass, landing gear extended), and continue on course (climbing turn) are well established (possibly even internationally?) and all pilots should know them, and are probably enough to defuse most situations. Not to mention that you should try to establish radio communications on 121.5 if you're being intercepted.
I'd wager a guess that the most common cause for interception is that a pilot has strayed into restricted airspace without talking to anyone on the radio, without any malicious intent.
If the pilot ignores the messages, there's always the option of using force, if the risk to occupants and collateral on the ground is acceptable.

Furthermore since flying in close formation to an unknown aircraft is far from standard practice, when someone (specifically a military craft) suddenly appears in front of you it's a pretty clear sign of "Hey you're doing something seriously wrong or suspicious," which in most situations is all that needs to be communicated, unless the pilot actually needs help. I'm not an aviation expert but I assume dispatching an escort is not a simple every-day occurrence (like for example a cop tagging cars on the highway) that can be ignored if you're not doing anything wrong.

They also sometimes intercept aircraft operating in the ADIZ just to verify that the tail number filed in the flight plan matches the actual aircraft that is flying.

You might also get "intercepted" if you decided you want to fly through an active MOA and the military aircraft decided they need something to do until you clear the airspace.

But why is there a specific frequency to communicate when you are intercepted? I would assume that modern fighter jets can listen/communicate to all channels at once and know which one the pilot is tuned to.

The AN/ARC-114A is a military airborne radio which covers the VHF band from 30 to 75.95 MHz and provides 920 channels at a spacing of 50 kHz. I think pilots general have more important things to do than monitor hundreds of channels. Imagine trying ot find your find in the field when you both have a walkie talkie with 920 channels and no agreed on convention. @TaherElhouderi

As far as I know, interception of civilian aircraft is done to assess a situation where the safe status of a flight is uncertain.
A Vueling A320 got intercepted some time ago when the French ATC weren't able to communicate with the pilot. The same airline some time later got another interception by the Dutch.
Intercepting the aircraft and watching the situation inside the cockpit and/or the cabin can help to assess whether there is a serious situation developing (and further actions can be taken, such as alerting emergency services on the ground, monitoring the flight path, continuing the interception procedure to try to improve the situation).
It's not about piloting the intercepted aircraft, but about assessing the status of the flight. I think it's just a matter of gaining some time if a hijacking occurs.

One large part of the reason to intercept any aircraft is to visually confirm  the identity of the aircraft.  While every other method of communication is valid in establishing the identity, only "eyes on" can unequivocally confirm the tail number and registration of the aircraft.

Well, they can confirm the tail number that is displayed on the aircraft.  Unfortunately, there have been cases (in drug smuggling airplanes for instance) where the tail number has been changed illegally.


text:
Why was a complete test flight of Hughes Hercules H-4 (Spruce Goose) never undertaken?
I get that ,the max altitude it was flown to was 70 ft. after its 3rd test taxi.I also am aware that even this was done to prove a point. But,why was there no attempt later on to complete a test flight and to check if it was Air worthy? Was no one curious at all ?

no-one wanted to front the money for the extensive test flights and see if it could fly away from the ground effect

By the time the government had pulled the plug on the program. Even that hop got Hughes into trouble because they had been expressly forbidden from taking off (he got off on a technicality, claiming it'd been unintentional, unexpected behaviour from an experimental machine, etc.).

If it had been anyone except Hughes I'm pretty sure the H4 would have been destroyed rather than preserved as a museum piece, the US government was that intent on killing the project there and then. They had a lot of air bases with long runways to find a purpose for, and long range air travel was an excellent purpose. They also wanted to find customers for the thousands of surplus transport aircraft (and bombers to be converted to that role). So getting rid of the traditional way to fly intercontinental, the flying boat, was important.

Were people curious? No doubt. But they also were budget conscious, the H4 had no more military application so there was no money coming out of the Pentagon. It had no civilian application, as for large payload ships are cheaper and for small, fast payloads it was too large. So even if there had not been active hostility towards the flying boat concept at the time from people with the power to release or deny budgets, there would have been little point in implementing an expensive flight test program (which usually requires multiple vehicles...) for an aircraft for which it was highly unlikely there would ever be any peacetime orders.

Of course 50 years later the idea was kinda revived with a short period of interest in Wing In Ground effect vehicles to fly troops and supplies over the feared Soviet submarine fleet in the Atlantic. But that didn't start up until near the demise of that threat in the 1990s and nothing ever made it past concept drawings (except paradoxically a few Soviet craft in the Caspian and Black Sea regions, the Ekranoplans).

I've little doubt the H4 would have worked as advertised, seeing it it looks sound in design, basically a scaled up B314. But there was just no market for it. It was an example of too big, too late.

Can you give the dates and maybe some references?

I get that the government was trying to pull the plug,but the aircraft was ready and had passed three taxing test-runs ,so why was no takeoff attempt made even after it was handed to a museum in pristine condition

@Qwe don't understimate the power of marketing & politcs. If politics used the excuse os unworthy to fly how they can look if they are proven wrong?

@Qwe the government owned the aircraft, not Hughes. The government grounded it, and didn't issue the paperwork that would have allowed its future owners to fly it even as an experimental.


text:
Could the US Air Force ever replace the Thunderbird pilots with drones?
Would it be possible or practical for the US Air Force to replace the Thunderbird pilots with drones?  The drones could perform more difficult maneuvers and do them more precisely.
The University of Pennsylvania is successfully working on small-scale formation flying.

Sorry, this is definitely an opinion storm in the making.  Personally, I'd say "what's the point of formation drone flying, it's easy".  VTC.

Sure... as soon as they're trying to recruit drones to join the Air Force, instead of people.

AFAIK, drones are ages away from being able to do formation flying at all, not to mention with the sort of precision decent human pilots can.

As a pilot, I view flight demonstration teams as a superb display of the skill of the pilots. Take the pilots out, and there would be no airmanship for me to be awed by. Perhaps if I were involved in the hardware and software development to enable drones to do that, I might then be able to generate more enthusiasm.

@JanHudec please define "ages", since this is already occurring on a small scale.  https://youtu.be/YQIMGV5vtd4?t=45s

@Simon I think the engineers working on the drone formation flying project above would argue with your assertion that it's easy.  I suspect it is actually hard.  Moreover, the complexity of the formation tasks could always be increased to make them hard.

@Terry the purpose of a warplane is not to exist for the pilot - rather the pilot exists to fly the warplane's military mission.

@TUMBLEWEED they could meet the engineers who create and maintain the technology.  There is always a human face behind technology when you dig deep enough.  Perhaps this would be a more relevant recruiting tool for a futuristic fighting force, rather than inspiring people to desire a job as a fighter pilot which is becoming less and less common.

@steampowered The Thunderbirds don't exist to fight wars. They exist to impress people. Drones are objectively worse at this than the badass military pilots with nerves of steel who can also stand and chat next to their planes before and after their performance. And they don't just exist to recruit fighter pilots; fighter pilots have never been the dominant portion of the air force. In fact, impressive stuff like that increases enlistment as well, and enlisted men aren't even eligible to fly planes. Basically, the Thunderbirds are just about the worst possible application of UAVs.

@steampowered "Less and less common"?  Would you mind quantifying your answer, since, to my knowledge, drones haven't replaced anyone yet.

@RhinoDriver I have a few friends flying the viper in the CAF, and they all tell me flying time has been getting more scarce between 2011 and 2015.  Meanwhile they are getting recruited into UAS jobs all the time.  I don't have a quantitative way to back it up, just what people say.  By the way, I half posted this as a joke - I don't like it either.  But everybody knows it's the future (even crusty old F-4 pilots, ha ha)

@steampowered Note that I prefaced my remark with "As a pilot...." Of course you are correct that the ultimate purpose of a warplane is not for the benefit of the pilots, but from my standpoint as a pilot enjoying a flight demonstration, my enjoyment is not based on the ultimate purpose, but on the piloting. When I was a 747 captain, the fact that I was getting 400+ people to where they wanted to go was incidental to the real reason I was doing it, that being because I loved to fly. Frankly, they could have paid me half of what they were paying me and I still would have done it.

@cpast, I do think selling recruitment to the modern USAF with F-16 flying in fingertip is most likely a relic from the 1950's which adds little value.  More importantly, it miscommunicates the opportunities which exist in the AF.  Fighter jets are being marginalized in the AF.  Both bomber and fighter roles will be replaced with robotic aircraft in the next 20 to 30 years.  Kids watching the thunderbirds think they are likely to work with F-16 flying in fingertip, but this is less and less the case.

@steampowered Meh, I've talked to engineers that are on the projects designing these drones, specifically the carrier variant, and they said there aren't even plans yet to design a strike fighter replacement. I don't doubt flying jobs become more scarce in a draw down, and with UAVs coming online I'm sure it looks like a 1 for 1 trade, but I think they are probably unrelated statistics. Especially considering there isn't an unmanned variant of the Viper anywhere to replace it.  That's not to say a predator can't perform some of the functions, but still, we're a long ways off.

@steampowered But I'm with you, drones are the worst.

@steampowered In the entire history of the United States Air Force, it has literally never been the case that fighter pilots were representative of the branch (nor are drone operators; most personnel are neither). The Thunderbirds do not exist to give a representative picture of the Air Force. "Fighter pilot" isn't even representative of the Thunderbirds; they have 8 pilots and over 120 non-pilots. The Thunderbirds exist to be cool; representativeness isn't the point. Drones simply are not as impressive. You can say they should be, but they aren't, and so it's a terrible choice for the team.

Incidentally, the USAF has 1,284 UAV pilots on active duty and 426 in the ANG (none in the reserves), according to their demographics query tool. They have 2,405 fighter pilots on active duty. There are 2,841 doctors on active duty. Just giving you an idea of how little the Thunderbirds are intended to represent the typical Air Force member.

@cpast trends and the rate of change of this value is more relevant than today's value.  I'm surprised the populations are that close.  I expect UAV/RPA population will pass other communities in the years to come.

@steampowered But they won't. Not anywhere enough to be "representative." More officers aren't pilots of any stripe than are pilots of any stripe. And 80% of the entire active duty Air Force are enlisted, who aren't even eligible to become pilots. Pilots are not and have never been the majority of the USAF. Drone pilots aren't and won't be. Since the Thunderbirds are about coolness and not at all about representativeness, there's no reason to care about representativeness of human vs. drone pilots.

Tight formation flying can be done with stock aircraft and skilled pilots, but drones would need additional sensors and new code in their FCS. Also, you can fly aerobatics even with gliders, but only when a decent power-to-weight ratio is reached, aerobatics becomes a spectator sport. I think it is frivolous for the US Air Force to pay for the development and additional equipment to make drones fly aerobatic displays, but few Air Forces are known for an enlightened budget discipline. I expect, however, that this will be added first by a company producing drones, and for demonstration and marketing purposes, and then the taxpayer will fund this indirectly.
Formation flying was a needed skill for bomber pilots, back then when their bundled defensive weapons provided the best protection against fighter attacks, and aerobatic flying was and still is valuable for fighter pilots. In the days of missiles and UAVs, neither formation flying nor aerobatics are necessary skills anymore, so in a way aerobatic formations are a quaint relic from the last millennium.
To answer the "could" aspect of your question: No, not directly. This would need modifications to existing drones.
To answer the "ever" aspect: Once the services field UCAVs (unmanned combat drones) with a good power-to-weight ratio, having them fly aerobatic displays looks almost unavoidable, but I expect it will not be the Air Force, but the marketing department of a big contractor driving this.

apparently the USAF is already flying F-16's unmanned  https://www.youtube.com/watch?v=ewoCUFne3MI

@steampowered … to then shoot at them, yes. Nothing new here.

@ Peter Kämpf   Remote controlling an aircraft for targeting in general at Tyndall is not new - RF-4 have been used for many years.  But the RQ-F16 is relatively new.   First one 2 years ago.

Like others have said, the Thunderbirds mission is recruitment. A drone formation is absolutely possible and would be quite the show, but it would not do much for recruiting civilians if the drones were the only part of the show.
Some might suggest using RPAs to bring a human aspect back into it, but that would not be a great idea either, at least with current technology. Formation flying relies heavily on recognizing small discrepancies and making small corrections to achieve that "locked" appearance. RPAs, like the MQ-9, have low latency if the operator and aircraft are in the same area, but there still is some latency with the signal being transmitted, translated, read, and displayed. In addition, unless the operator is wearing some sort of augmented/virtual reality headset and the aircraft has some 180+ degree camera, the spatial awareness will be severely degraded. Finally, our current RPAs are loiter aircraft, so they are really not fit for being in formation. What I mean is that the wide wings and light-weight design make the aircraft prone to the effects of turbulence, convection, probably even the propwash of the other aircraft in formation.
All that being said, I think once we get the jet-propelled drones more developed and trusted, we might be seeing them in airshows. The MQ-25, a refueling, carrier-based drone, would be a prime candidate for a refueling demo (the ones where the fuel recipient isn't actually connected).

Who said anything about replacing them?  Why not have both?  For lower budget, smaller shows, send in the drones!  Absolutely love the formation photo.  Why not?

@RobertDiGiovanni , the original poster alluded to replacing them.

Copy that,  I am offering an alternative.

Then the phrase is "Who says we have to replace them?", not "Who said anything about replacing them?". Nobody said we have to replace them, but OP clearly brought it up as a possibility.

I like the design so much, why not put pilots in them?

UAVs and UCAVs can be programmed to fly in formation as has already been demonstrated. There may be military applications for this capability. A tactic used by Luftwaffe "sturmgruppe" units against US bomber formations in World War Two was the "company front" assault in which the heavily armoured FW190 fighters, flying in a close line abreast formation, would approach a bomber formation from the rear, dividing the defensive firepower, and then break up at the last moment to attack individual bombers. UCAVs could potentially use similar tactics effectively against enemy ground, naval or air forces in some situations.


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How are wind tunnels used with scale models?
This answer introduced me to the problem of using scaled models in a wind tunnel.
How are wind-tunnels used in practice (for modelling the performance of subsonic airplanes)? In particular:

Do they used increased air pressure, or another mechanism, to compensate the Reynolds number?
What scale model is used? A scale of e.g. 70:1 appears to be difficult, because you can't pressurize air that much before it liquifies. So do they use a scale like 10:1? More or less?
What are the forces (lift and drag) generated on the model? The weight (therefore the lift) of an A380 is about 500 tons. Apparently the lift on a 10:1 scale model would be 50 tons. Is that real? Are they making 10:1 models which cope with 50-ton forces?
This picture (from Wikipedia's Wind Tunnel article) shows what seems to me to be a 10:1 scale model of a supersonic plane in a subsonic tunnel. The tunnel doesn't seem to me to be built to handle multi-ton forces. What kind of air-speed will they be using, what would they be measuring with that kind of test?

This is a lot of questions at once, perhaps you could separate them in different questions? For example one asking about the reynolds number, and one about the lift forces

You might be right, OTOH perhaps each question-mark might be answered by a single sentence. I think they're closely-related, maybe one topic.

I'd guess they are mostly investigating things that don't depend that much on the reynolds number.

@ChrisW Well I was about to write a story about Reynolds numbers, but then I saw the other questions and decided not to ;)

During development, a lot of different wind tunnels are used. Scales range from full scale to less than 1/100, and sometimes not only the geometric, but also the dynamic and elastic properties of the aircraft to be tested must be modelled correctly.
Today, preliminary work is mostly done in numerical simulations, but before the availability of computers, early wind tunnel models would be between 1/50 to 1/16 scale (depending on the size of the real article and the available wind tunnel), often using modular models on which different engine placements or tail geometries could be tried out. Since wind tunnel testing costs real money, early tests are restricted to small, low-cost tunnels which run at ambient temperature and density. Differences in the Reynolds number between test article and the real aircraft are covered with correction factors and experience.
Only when a design is moving ahead during development, bigger and more expensive tunnels are used. Again, matching the Reynolds number is not possible and not even necessary for some tests: For a spin tunnel the test article must be scaled dynamically (so it has correct moments of inertia), but separated flow is less affected by the Reynolds number, so the precision of a 1/20 scale model is sufficient. Also, a free-falling model in a spin tunnel needs quite a bit of space to move around, so even small models already need test sections with a diameter of several meters.
To reduce cost and avoid the tight schedules of wind tunnels, free-flying or radio controlled models are also used. Watch how Dornier engineers tested the dynamic behavior of the then-new tricycle gear of the Do-335 in this video (starting at 2:00') or ditching characteristics (starting at 2:13').
Only when both Mach and Reynolds number need to be matched, cryogenic and pressurised wind tunnels are used. Due to their energy demand and availability, such tests need to be scheduled years in advance and meticulously planned. Models can run into millions of dollars or Euros, so such tests will mostly narrow down parameters which had been measured previously in simpler tests. Large tunnels like the 9.5x9.5m LLF in Marknesse (the Netherlands) are mostly used for low-speed and high-lift characteristics testing since its 12.6 MW engine can only support a speed of 62 m/s at full power. On the other end of the scale are hypersonic blow-down tubes which support a testing section of 0.5m diameter in which Mach 6 can be achieved for a fraction of a second.
Full-scale testing in a wind tunnel is only possible with small aircraft - see the answers to this question for examples. Even then, the speeds do not match the top speed of the aircraft, so the loads are a fraction of the real loads.
Jan Roskam observes in his book "Roskam's airplane war stories":

It certainly is an unusual luxury for an engineer to have full
  Reynolds Number tunnel data available.

62 m/s is 120 knots which is barely take-off speed for a jetliner.

@ChrisW: Yes, and at the reduced scale the Reynolds number is a tenth or less of what it is in real life. Accordingly, the gaps between flap segments are proportionally bigger on a wind tunnel model to achieve the same flow at high angle of attack. You don't need to match Re exactly, it is enough if you come close and design the model accordingly.

For me your last link ("Roskam's") displays a blanked-out page with the message, "du hast entweder eine Seite erreicht, die nicht angezeicht werden kann, oder die Anzeigebeschrankung fur dieses Buch erreich." It would be better to post a screenshot of the page, as I did in this question.

@ChrisW: I just googled the phrase and that is what came up first. I guess Google is picky about what to show - in Google books many pages will not be displayed. I replaced the link with one to the publisher, but recommend to shop around. My copy of the book is the Kindle edition.

Usually, for low subsonic cases, Reynolds number is matched, while for transonic and supersonic cases, the Mach number is matched.
In case you only want the Reynolds number similarity, you can go for a water tunnel.
The only way to get both Reynolds and Mach number similarity for models (unless you change the medium) is to use pressurized and/or cryogenic wind tunnel. The cryogenic wind tunnel achieves the required Reynolds number by reducing the viscous forces rather than increasing the inertial forces; even then the actual Reynolds numbers encountered in flight are rarely achieved and one has to resort to scaling laws.
The scale of the model used depends on the application. In some cases (like hypersonic vehicle testing) 1:1 scale models are used, while in other cases like A380 testing, even 200:1 models were used.
Though the force acting on the models would differ by applications (and even approach the actual forces in some cases), the forces acting on the smaller scale models are quite small (note lift and drag scale as square of the dimension provided other parameters are same, which is not even the case here). In general, various scaling laws are used to arrive at the forces experienced by the actual aircraft. For example this video shows the forces in an A380 scale model in the order of Newtons. Usually, you would like to find the lift (or drag) coefficient of the model and use it in actual case.
The Kirsten Wind Tunnel in the image you've linked can measure a maximum lift of slightly more than a ton with the other forces an order of magnitude less than that. Note that this is a quite large subsonic tunnel and smaller tunnels can measure even less. You have to scale the results according to the model. The air speed is given as ~ 90 $ms^{-1}$.
There are a number of things measured and observed in the wind tunnel, which may only be loosely related to force measurements like flow visualization, effect of store jettisoning, wake analysis, among others.

That A380 test video, for example, which uses a tiny model in a low speed wind tunnel: I guess it didn't try to match the Reynolds number. If it doesn't match the Reynolds number, aren't its measurements completely useless (inaccurate)? Because the Reynolds number affects turbulence (laminar separation over the wing surface etc.), and turbulence is very important?

1:1 models for hypersonic vehicle testing? I always though hypersonic wind tunnels were shock tubes with test sections smaller than 0.5$m^2$. Do you have any examples?

Thanks for the other  answers. The last part of my question, i.e. what's the point of testing using small models which don't match the Reynolds number, seems to be answered by the graph in  this answer:

Lift coefficient over angle of attack for model and full-scale aircraft, taken from Joseph Chambers' monograph on testing with models (Modeling flight : the role of dynamically scaled free-flight models in support of
NASA’s aerospace programs).
This graph shows that the lift coefficient of the model and the full-scale aircraft are identical at ordinary angles of attack; they diverge at steeper angles (maximum lift), as stall becomes imminent (the model stalls more easily); and become identical again post-stall. Even when they're at their furthest apart they're not very different (e.g. the difference is a factor of 2 rather than a factor of 50).
The paper this graph was taken from has a wealth of other information.

Also this video from the European Transonic Windtunnel explains how they achieve realistic Reynolds numbers: by lowering the temperature (e.g. to 110 K, i.e. -160 °C) by evaporating liquid nitrogen after the compressor, without oxygen (which would liquefy), and by increasing the pressure (up to 4.5 bar).
It claims 99% (and better) accuracy.
The largest models it can fit have a wingspan of about 1.6 metres.


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What happens when a propeller is placed in beta while feathered?
Could someone explain beta range to me?
What happens when we set the throttle control lever to the beta range (reverse thrust) while the propeller blades are in the feathered position?
Will the blade angle change from feather to reverse pitch or will the engine over torque?

Throttle doesn't go negative, you set beta with pitch. This is controlled automatically when going into beta by the pitch control unit. Unless you are talking about an aircraft with full manual controls like some GA float planes? Do you have a specific aircraft you are referencing?

I am currently working with Cezh Made LET 410 UVP -E20 aircraft. But I was asking this question in general.
As far as I understand; its the throttle lever used which in turn control the PCU to bring the propellers to the Reverse Pitch right?
What I am asking is your Power is Idle and blades are in feather. What will happen if you pull back your Throttle Lever to Beta range?

The PCU commands the prop into beta pitch.

Thank you. So the PCU can sufficiently change the pitch from Feather to Fine and then Reverse.

Depends on they type of turboprop engine.  With a straight shaft turboprop engine like a Garrett TPE331, this is not possible as feathering the props are controlled by the conditioning levers and also shut down the engines in the process.  A free turbine turboprop would allow a person to feather the engines and then pull the power levers to the reverse position.  Note that the propeller lever control does not set beta here, rather the power levers do, moving from idle to beta to reverse.


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Aircraft Identification: What single-engine, T-tail, commuter aircraft has a single prop but sounds like a jet?
My morning walk takes me past Hillsboro Airport, Portland, Oregon (HIO), and today I saw a rather strange plane landing.  It was a small commuter-size plane (6 passenger windows on each side), with a single surprisingly large propeller on the nose.  While landing it sounded much more like a jet than a prop.  The tail was "T" shaped and I thought I saw a jet exhaust at the back of the fuselage.
Was I imagining this, or does such a craft exist?
Sorry I didn't get a picture.

I edited to include the name of the airport. Of course, the IATA code uniquely identifies it but nobody outside the immediate area is going to know the IATA code of some small municipal airport.

Sounds like a Pilatus PC-12 to me.

image source: Wikipedia, photographer: Chris Finney

That's definitely it, thanks. I guess I imagined the exhaust at the back since the turbine exhausts seem to be at the front behind the propeller.

What is the small pod at the right wing tip?

@mins that is the weather radar.

If this is not a PC-12, then this is a secret prototype of the Cessna Denali :-)

Could have been also a TBM 850, which is very similar.

@kebs No, it had more windows and the tail was definitely a "T" shape.

@kebs That's a new answer - avoid answering in comments.

@Criggie can you add the credits/source for that image you added?

@DeltaLima Done - its in the review queue now.

cool answer to a cool question.


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Can a 40 hp ultralight trike with a 15 m wing take off with simple (homebuilt) tube floats?
There seem to be no floats manufactured for 103 trikes- under 500 pounds including gas and pilot. Seems like they could be smaller and lighter than existing ones.
I am considering fabricating floats primarily for a possible emergency engine out landing along a rocky coast.  Being able to take off would be great, but secondary.  (I would prefer to not turn turtle and sink instantly). I would be landing at about 25 mph
Can a 40 hp trike with a 15 m wing take off with simple tube floats or are stepped hard shell stepped floats necessary?

Define "tube floats". There are tubes made out of soft rubber, like tire tubes, and there are floats made out of aluminium but shaped like tubes, used in some types of inland watercraft. ?? Pictures of what you mean would be very helpful. 8)

If it is not required to take off from the water and it is only for emergency landing then why would you want to deal with the  drag the tubes produce? I'd say that the primary aim is not to loose the trike by sinking. So I'd use (one or two) inflatable life wests mounted on the trike that you can use after landing to make it float.

The floats would be hard shell fiberglass similar to a paddle board. I am wondering if floats on a weight shift trike (with the weight hanging freely below the wing ) would be "stuck" to the water and need the step on the float.


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Can I take the FAA CFI knowledge exam(s) based on my EASA ATPL knowledge?
One of the requirements to be a flight instructor in the USA is FAA theoretical exams.
Can I sit those exams with EASA ATPL theory basis?  Also, do I have the right to sit those exams without completing an official FAA theory exam preparation process?

There is no official theory exam preparation process that is required to be able to take the test, you can do all preparation on your own. If you are asking if having passed the EASA ATP theory exam means you don't have to take the FAA CFI then the answer is no, you definitely would have to take the exam, although I don't think that's your question.
I think what you're asking is if the material in the EASA ATP is the same in the FAA CFI and you don't need to study. While the material is very similar and there will be a lot of overlap the two are not the same, there will be questions which ask specifically about FAA regulations, testing standards and student training which are not obvious and you are not likely to pass without knowing the material.

You are going to have to be a little bit more clear with your question. I don't know exactly what you are asking.
With that said, for the CFI-Airplane initial practical test, you will have to have taken the CFI Aeronautical Knowledge Test (I hope I have the current wording correctly).  As far as I know, there are no substitutions, and you must take that test.
Additionally, you will need to take a Fundamentals of Instruction test, unless you already have a Ground Instructor certificate or you are an accredited teacher. Around here if one is "licensed" to teach K or 1st grade, that is considered adequate to waive the FOI test.
The knowledge for a CFI is quite different from the knowledge needed for an ATP (while ATPs may teach in their work).  Furthermore, the expectations and rules for CFIs vary between countries.  I know Canadian CFIs who have had to struggle with the US CFI coursework, despite their obvious competence and excellent track record in Canada.
One more item worth mentioning: The ability to answer as well as ask questions in a clear and unambiguous manner is an expectation of CFI's in the US.  So you will want to hone your skills a bit, based upon the clarity of the question you asked.

The testing requirements are spelled out in 14 CFR §61.183(d)(e)(f), which require you to received a logbook endorsement from an authorized instructor to take the fundamentals of instruction knowledge exam.  These requirements can be waived if you are currently employed as a teacher in junior high or high school or are a college professor.  The only other requirement is that you pass an aeronautical knowledge exam appropriate to the category and class of aircraft you are applying for on a flight instructor’s certificate.

(d) Receive a logbook endorsement from an authorized instructor on the fundamentals of instructing listed in §61.185 of this part appropriate to the required knowledge test;
(e) Pass a knowledge test on the areas listed in §61.185(a)(1) of this part, unless the applicant:
(1) Holds a flight instructor certificate or ground instructor certificate issued under this part;
(2) Holds a teacher’s certificate issued by a State, county, city, or municipality that authorizes the person to teach at an educational level of the 7th grade or higher; or
(3) Is employed as a teacher at an accredited college or university.
(f) Pass a knowledge test on the aeronautical knowledge areas listed in §61.185(a)(2) and (a)(3) of this part that are appropriate to the flight instructor rating sought;

As to specific preparation for the flight instructor aeronautical knowledge exam, there is no special requirement for this and It does not require a formal ground school or home study course nor an instructor endorsement to take it.  You will be required to receive an instructor sign off to take the fundamentals of instruction exam, and a formal study course is recommended as preparation for this.
I’d also recommend a formal ground school with experienced instructors for preparation to obtain an instructor rating.  Having the knowledge of how to fly and airplane is one thing.  Being able to teach another person how to fly is an entirely different animal.  Some formative tutelage by an experienced instructor on all the knowledge and practical nuances required for an instructor certificate can go a long way.  The FI and CFI airplane knowledge exams aren’t bad, but the CFI practical test is going to be pretty grueling.  Mine was 10 hours long - 7.5 oral, 2.5 flying - and probably the most difficult exam I ever had in my life.

You don't need an instructor's endorsement to take the FOI test; I did it without one, for example. The FAA's written test requirements appears to confirm that (see p.10).

@Pondlife I too took the FOI and CFI tests without an instructors endorsement.

If you did, it was breaking the law.  See the FAR segment above.

I'm not sure how it was breaking the law? 183(d) requires an endorsement, but it doesn't say that it's a prerequisite for the written. You can compare the wording in 103(d)(2), which does make it a prerequisite for private: "Certified that the person is prepared for the required knowledge test". The FAA's guide is extremely clear that no endorsement is needed, and the test center checked the prerequisites carefully too.

@Pondlife straight out of AC 61-65:  The endorsement reads “I certify that [First name, MI, Last name] has received the required fundamentals of instruction training of § 61.185(a)(1). I have determined that [he or she] is prepared for the Fundamentals of Instructing knowledge test.”  So yes this endorsement is directed at an instructor acknowledgment that the individual is ready to take the FOI.


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How does Reynolds Number affect skin friction drag?
I'm an aerospace engineering student, and I'm worried about efficiency as it relates to high and low Reynolds numbers. I don't understand which is more helpful for a aircraft.
On a website I read this:

If the Reynolds Number is large, the viscosity effect is small. For the for us practical values the inertia or density forces dominate, and the parasite drag increases with the square of the velocity. However, although the viscosity is unimportant, it may still affect the very thin boundary layer, leading to the creation of turbulent flow.
  Thus the importance of the Reynolds Number is that it tells us the type of flow we can expect. It tells you whether you can hope for having laminar flow over the wing and other parts of your airplane. A low Reynolds Number gives laminar flow while a high Reynolds Number gives turbulent flow. For both a laminar and a turbulent boundary layer increasing Reynolds Number gives lower skin friction drag. However, because of the higher energy loss in the boundary layer, a turbulent layer always has higher skin friction drag.

In this, there is something like this: For both a laminar and a turbulent boundary layer, increasing Reynolds Number gives lower skin friction drag. I didn't understand the meaning of that statement.
Is there a rule like "Higher Reynolds number, higher drag force"?
And what exactly do parasite drag and boundary layer do?
And what do their vastness or rarity contribute to?
Simply: how big should a Reynolds number be for an aircraft to be efficient?

First of all, friction is bad for efficiency. It slows moving things down and needs a constant energy supply to be overcome. The more friction, the more energy has to be supplied, and this energy is lost as heat.
Aerodynamic friction is caused by viscosity. The Reynolds number tells you how big viscosity is in relation to inertial forces. A bigger Reynolds number signifies lower viscosity. This means a higher Reynolds number almost always results in lower friction. If you look at the plot below, the downward trend can be easily spotted.

Friction drag coefficient of a flat plate over Reynolds number (picture source). Note the double logarithmic axes.

Is there something like " higher Reynolds number, higher drag force " stuff ?

Yes, sometimes. Please look at the transition curve: Here a flow which is initially fully laminar slowly changes with increasing Reynolds number into one with a laminar start and a transition to turbulent flow somewhere downstream. This adds a section of turbulent boundary layer where at a lower Reynolds number the flow would had been laminar. You will notice that for the same Reynolds number, a fully laminar flow (lower line) is much less draggy than a fully turbulent one (upper line). Therefore, replacing some of the laminar flow with a turbulent one will result in more drag.
Now I need to explain why increasing the Reynolds number leads to more turbulent flow. For this, I turn to this answer:
Inside the laminar boundary layer, small disturbances become less and less damped the higher the local Reynolds number becomes, and at a Reynolds number of around 400,000 in unaccelerated flow some frequencies become unstable (see Tollmien-Schlichting waves) and will eventually create so much cross movement that the boundary layer becomes turbulent. Now parcels of air which flow at high speed in the outer part of the boundary layer will move close to the wall and kick the slow parcels there ahead, greatly reducing the deceleration of the flow close to the wall, at the price of slowing down and expanding the whole boundary layer.
Simply put: A laminar boundary layer cannot be sustained indefinitely but will change to a turbulent one if the Reynolds number is high enough. This will swap some of the previously laminar portion of the boundary layer to a turbulent one, increasing drag.
While all of the above is only strictly true for flow over a flat plate without a pressure gradient, real flow will complicate things again: Now turbulent flow is not all bad; the example of the dimpled golf ball shows that a turbulent boundary layer in flow with a pressure gradient can also lead to lower drag, given the right circumstances.

@ymb1 Turbulent boundary layers "borrow" energy from their outer fringes and transport it to the wall. By kicking the laggards close to the wall forward, this mechanism decelerates the outer part of the BL, making it thicker in the process. While laminar BLs only exchange energy perpendicular to the flow by friction, turbulent BLs transport the moving air around the BL, so the energy exchange perpendicular to the flow direction is much higher. And there is no energy to be added – as Homer Simpson says: We obey the laws of thermodynamics!